The ability of transition metal ions to undergo facile 1-electron oxidation or reduction makes them obvious potential chemical partners for reactions involving biological free radicals. It is not coincidental that superoxide dismutases, the enzymes that catalytically destroy the superoxide radical (O2) by alternately oxidizing and reducing it, have been found containing 3 different transition metals (Cu, Mn, or Fe) at their active sites. Iron is by far the most abundant transition metal in the human body because of its roles in oxygen binding and transport and electron transport. Because of the central and essential roles of iron in the metabolisms of all aerobic organisms, humans have evolved some peculiar ways of dealing with it. These peculiarities provide opportunities for the cause of diseases related to iron absorption, transport, and metabolism, as well as for the exacerbation of general mechanisms of disease involving free radical injury.